Controlled FTIR measurements of acid gas (NO, mixed NO/SO 2 ) capture in water at high pressure

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ANLECR&D project 6 0215 0234 Controlled FTIR measurements of acid gas (NO, mixed NO/SO 2 ) capture in water at high pressure Sub contract to

Emissions on depressurisation, stability, disposal and utilisation potential By Peter F Nelson 1, Rohan Stanger 2 and Terry Wall 2 1

1 ANLECR&D project Controlled FTIR measurements of acid gas (NO, mixed NO/SO 2 ) capture in water at high pressure Sub contract to Macquarie University Report of ANLECR&D Project on: Products formed from gas impurities in oxyfuel derived CO 2 compression: Emissions on depressurisation, stability, disposal and utilisation potential By Peter F Nelson 1, Rohan Stanger 2 and Terry Wall 2 1 Department of Environmental Sciences, Macquarie University, NSW Australia 2 Chemical Engineering, the University of Newcastle, NSW, Australia March

2 Executive Summary Additional experiments made on the UoN small pressure rig were undertaken with NO x and mixed NO+SO x injection using a combined gas analysis system (FTIR/NO x /N 2 O). This complementary work was made because industrial NO x analysers do not differentiate NO 2 from other forms of NO x (HONO, HNO 3, N 2 O). The intention was to provide tightly controlled gas flows to enable the FTIR identification and quantification of other species formed during the compression process. It was found that the variable amounts of water vapour had an over riding influence on baseline FTIR measurements which required specific deviations from previous methods. Overall, this work has identified N 2 O being formed during the injection of a mixed acid gas stream in similar quantities to those measured by the dedicated N 2 O analyser. It has also confirmed the presence of HONO released during de gassing of the liquid phase from a 25 bar condition. However, the trace levels (~1.3ppm) indicate that the measured NO and NO 2 traces are likely real gas concentrations evolved from desorbing out of the depressurised liquid. This also suggests that the 1 hour residence time at 25 bar is sufficient to oxidise dissolved HONO formed in the NO x absorption process. The FTIR also confirmed the presence of HNO 3 desorbing out the back sections of the reactor at concentrations similar to those measured with the NO x analyser. The presence of HNO 3 represents a potential safety hazard during maintenance of an oxyfuel compression circuit. 2

3 Introduction The Callide Oxyfuel Project CO 2 Processing Unit (COP CPU) was designed to remove SO x at atmospheric pressure using a 2 column caustic scrubbing unit (shown as Scrubber 1 in Figure 1). The removal of NO x occurred at higher pressure during compression and in a high pressure column (Scrubber 2 in Figure 1). However, other gas vendors have proposed alternative circuits involving co removal of SO x and NO x at high pressure as a method of improving overall capture and reducing CPU footprint. Air Products Sour Gas Compression process (Figure 2) involves high pressure co removal of SO x and NO x in a 2 column system operated at 15 and 30 bar, respectively. The application of this process is suited to Australia, as both NO x and SO x are present at reasonable levels in flue gas in Australian coal fired power stations, and their co removal avoids the cost of the addition of separate SO x removal technologies. Capture at higher pressure also provides a method of reducing overall unit size and thus capital requirements. Notionally, the presence of NO 2 also acts as an oxidiser for SO 2, providing greater SO x capture. This mechanism is based on the lead chamber process used in sulfuric acid manufacturing. However, the co capture of SO x and NO x at atmospheric pressure has been linked with the formation of N 2 O a potent greenhouse gas equivalent to 300 the impact of CO 2. The current work extends this finding to high pressures. The UoN has previously reported on the formation of N 2 O during co removal of mixed acid streams on both the small pressure system and the bench scale compression unit [2]. In that report the presence of N 2 O was measured using a dedicated industrial N 2 O analyser. This work reports on additional experiments made on the small pressure system which included the use of an FTIR spectrometer to alternatively verify the presence of N 2 O and estimate its concentration. This combined analytical approach was also extended to NO x only measurements (ie no SO 2 injected) made under controlled conditions to quantify other nitrogen containing species present in various parts of the experimental system. Identification and quantification of nitrogen containing species observed in experiments at the University of Newcastle at high pressure from the compression of oxyfuel gas were described in our previous report [3]. Importantly, the use of an industrial chemiluminescent NO x analyser measures the presence of NO and by difference through a convertor Total NO x. Typically, the difference between NO and Total NO x is attributed to NO 2. However, our research has suggested that this measurement may also include other nitrogen containing species HNO 2, HNO 3 and N 2 O, depending on the experimental conditions. Here we report additional laboratory FTIR measurements to the presence of nitrous oxide (N 2 O) in the products of reactants comprising 1000 ppm NO, 1000 ppm SO 2, at 5 bar, and in the presence of water. In addition further experiments which attempted to identify water soluble de gassing products were also undertaken. 3

Figure 1. Air Liquide s CPU design for the Callide Oxyfuel Process with atmospheric scrubbing of SO 2 followed by high pressure removal of NO x. Figure 2.

The mechanisms of NO x transformations and capture have been studied extensively by the UoN, in part as they relate to mercury capture prior to the brazed aluminium heat exchanger in the COP

4 Figure 1. Air Liquide s CPU design for the Callide Oxyfuel Process with atmospheric scrubbing of SO 2 followed by high pressure removal of NO x. Figure 2. Air Products Sour Gas Compression process with co removal of SO x and NO x at high pressure using two columns at 15 and 30 bar respectively (taken from [4]). The mechanisms of NO x transformations and capture have been studied extensively by the UoN, in part as they relate to mercury capture prior to the brazed aluminium heat exchanger in the COP CPU. Figure 3 shows a summary of these processes presented in an invited webinar by the Global CCS Institute in Importantly, it shows the challenge of obtaining a nitrogen mass balance and the need for different experimental measurements to capture these impacts. 4

5 NO NO 2 Figure 3. High pressure mechanisms of NOx and mercury capture during oxyfuel flue gas compression showing capture in condensates (as stable nitrites) and as acid condensation. Dissolved gases may potentially be an emission when condensates are depressurised, as indicated by the experimental evidence. Taken from [5]. Experimental The small pressurised reactor used at the UoN for COP CPU mechanistic studies has three modes of experimental operation (Figure 4); these being (1) Absorption of NO x into liquid water at high pressure; (2) de gassing of the liquid after the experiment and (3) desorption of condensed acids from the back end of the reactor after the experiment. (1) Absorption by liquid (2) De gassing of liquid (3) Desorption of condensed acid NO x (+SO 2 ) N 2 5% O 2 NO x (from surfaces) H 2 O H 2 O bypassed Air NO x (from liquid) Figure 4. Diagram showing the modes of experimental operation showing (a) absorption of NO x (and/or SO x ) into liquid water at pressures; (b) de gassing of volatile NO x species out of the liquid after absorption and (c) desorption of NO x species from the non wetted surfaces after the reactor (bypassing liquid). 5

6 Mode 1 (absorption) involves the initial contacting of NO x gases with liquid water in which capture occurs through the gas phase oxidation of NO to NO 2 and the subsequent absorption of soluble NO 2 as a mixed acid (HNO 2 /HNO 3 ). Higher pressure has been shown to increase the oxidation of NO and improve overall NO x capture. Previous mixed acid work has shown that the addition of SO 2 injected into the small pressure system results in the formation of N 2 O at 5 and 25 bar to different extents. This work replicates those experiments at 5 bar with FTIR measurements as verification. Mode 2 (de gassing) involved the controlled removal of the liquid after absorption of NO x only (over ~1 hour at 25 bar). The liquid was dropped out in 40 50mL aliquots into a sealed vessel with a constant flow of air at 3LPM. Dissolved gases present in the liquid were then allowed to de gas over 1 hour before the next aliquot was taken. It was hypothesized that the absorption of NO 2 into a mixture of HNO 2 /HNO 3 would result in a build up of HNO 2 dissolved in the liquid phase (HNO 2 being fully ionised). The FTIR measurements taken during this period were averaged over several minutes to obtain sufficient signal. This is important as much of the dissolved gases were observed to be removed over the course of several minutes and hence much of the transient changes are averaged out by the FTIR. Mode 3 (desorption) involved bypassing the liquid section of the reactor and depressurising the transport lines. This allowed for the dissolved gases to be contained in the reactor while the backsections of the system could be flushed. Importantly, this mode of analysis has previously measured a large peak in NO 2 (of the order of several 1000ppm) over the course of several minutes. It was hypothesized that the gas phase formation of HNO 3 in the back sections of the system could occur because of the presence of water vapour derived from the reactor. Once condensed onto the surface, the lack of a liquid water phase could allow the HNO 3 to retain sufficient vapour pressure to be evaporated once depressurised. The FTIR experiments were taken after a 1 hour absorption of NO x only injection at 5 bar. FTIR Operation The FTIR spectra in this series of experiments, performed in November 2014, were collected under the same conditions as previous NO x only experiments. The outlet flow of the high pressure batch reactor at atmospheric pressure (ie after reduction of the reactor pressure) was connected to a Nicolet 6700 FTIR. The FTIR cell was operated in flow through mode (the flow was determined by the conditions of the experiment) in order to minimally perturb the concentrations of reactive nitrogen species measured in the products and to avoid the condensation/ disappearance of these species. Specific to these experiments was the need to provide the FTIR analyser with a sample gas with constant H 2 O vapour. This required a minor change to previously reported work, where the experiment was started by bubbling N 2 only through the pressurised system. Once a H 2 O baseline was established the NO and SO 2 gases were turned on through custom designed Mass Flow Controller (MFC) software. A folded beam gas cell with a total path length of 2.4 meters (Infrared Analysis Inc., Model 2.4 H) was custom fitted to the FTIR. The FTIR was operated using a liquid nitrogen cooled MCT/A detector with the following beam parameters: 128 scans were co added to obtain spectra at 0.25 cm 1 resolution with a scanning range between cm 1. Spectra were recorded using FTIR software (Thermo Scientific Nicolet OmnicTM). 6

7 Peak identity was confirmed and concentrations estimated by comparison with standard spectra provided by a Database of 386 Digitized Quantitative Gas Phase Reference Spectra (Infrared Analysis, Inc QASOFT Package 4.0). Results (Mode 1) Confirmation of nitrous oxide (N 2 O) formation from mixed acid gas (SO2/NO) injection This experiment was performed 18/11/2014 under the following conditions: flow 3 SLPM, 1000 ppm NO, 1000 ppm SO 2, 5 bar, and 150 ml water in the bubble reactor. Figure 5 shows FTIR spectra for the frequency range of cm 1 at three times after the commencement of this experiment, and also for the feed gas. All show the characteristic features for gas phase spectra of N 2 O. The feed gas for this experiment shows small amounts of N 2 O (~10 ppm) probably as an impurity in the NO gas used. Each of the product gas spectra show significantly enhanced concentrations of N 2 O: 200 ppm about 2 3 minutes after the commencement of the experiment, increasing to 240 ppm once steady state conditions had been reached. This matches measurements of N 2 O made in UoN Milestone 4.1 report (with a dedicated N 2 O analyser) which showed a peak N 2 O concentration of 230ppm with the same feed conditions. The observation of N 2 O formation under these conditions is not unexpected given previous research [6] which showed significant amounts of N 2 O formed from coal combustion gases containing similar amounts of NO x and SO 2. As N 2 O has a global warming potential times that of CO 2 for a 100 year timescale, the formation and fate of this species in full scale oxy combustion systems requires investigation and evaluation. 7

8 0.4 Absorbance a b c d Wavenumber (cm -1 ) Figure 5: FTIR spectra of the region of N 2 O absorbance for feed and product gas from 5 bar experiment in University of Newcastle pressure reactor. (a) product gas spectrum 2 3 minutes after commencement of flow; (b) & (c) product gas spectra under steady state conditions. (d) feed gas spectrum with N 2 made prior to acid gas injection into 150 ml water in bubble reactor, showing trace N 2 O impurity. 8

9 (Mode 2) Species observed during de gassing of liquid water after NOx only absorption at 25 bar This experiment was based on replicating previous NO x only measurements made on the small UoN pressure system. Initially, experiments with the FTIR attached were made to confirm the presence of other NO x species (Nelson and Bray, 2015). In replicating the work, a more controlled set of measurements could be made to quantify the concentrations of these other NO x species, specifically HONO and HNO 3. Spectra were collected at a range of times after a 25 bar experiment with NO injected at 1000 ppm, O 2 set to 5% and collection of potential products in liquid water. Specific to this experiment was the addition of liquid water in the degassing vessel, to ensure that the H 2 O vapour was constant. The pressurised liquids were dropped out into a smaller beaker sitting in this water bath. Figure 6 shows the FTIR product gas spectra from this experiment. Trace concentrations of N 2 O of ~1.3ppm are observed but no nitric acid (compare the large amounts of nitric acid observed in Figure 19 of our previous report [3]). The NO and NO 2 measurements taken with the NO x analyser showed that NO 2 reached a peak of 35ppm during these de gassing experiments. This suggests that the high pressure conditions produced a relatively stable liquid. Previous work had confirmed the presence of HONO (ie nitrous, sometimes written as HNO 2 ) during volatile release experiments, leading to the conclusion that it was the main volatile species released during de gassing. However, these controlled experiments have allowed quantification of HONO concentration at trace levels. This suggests that the main volatile species evolved from high pressure liquids are in fact dissolved gases NO and NO 2. 5 Absorbance Wavenumber (cm -1 ) Figure 6: Product gas spectrum in region of HONO and HNO 3 absorbance from collected water experiment; features due to trace HONO (~1.3ppm) observed but no HNO 3. Figure shows the spectra at intervals after the commencement of the desorption phase from the collected water. These spectra are complicated by the very large absorption due to gas phase water but Figure (a) and (b) show evidence of very broad absorption. It is not clear what species might be responsible for these features but water condensing in the FTIR cell is one possibility. 9

10 a 2.0 Absorbance b c Wavenumber (cm -1 ) Figure 7: Spectra collected from collected water during desorption. Sharp lines due to gas phase water. (a) to (c) spectra collected at intervals of ~5 minutes at increasing times after commencement of desorption 10

(Mode 3) Species observed during acid desorption test after NO x injection at 5 bar As with Mode 2, these experiments were designed to replicate previous NO x only measurements.

11 (Mode 3) Species observed during acid desorption test after NO x injection at 5 bar As with Mode 2, these experiments were designed to replicate previous NO x only measurements. The acid desorption at the end of the NO x injection experiments has previously been observed as a NO 2 peak well above the measurement range of the NO x analyser. It was theorised that this material was HNO 3 produced in transport lines downstream of the pressure vessel (in the presence of H 2 O vapour) which was subsequently desorbed into the gas phase. In this experiment, the NO 2 was observed to peak above 500ppm, then drop to 20ppm within 20 minutes. A smaller secondary peak was then observed to occur over ~45 minutes. The purpose of these measurements was to identify that this species is HNO 3 and quantify its concentration. Figure shows spectra obtained during acid desorption. The time elapsed from the beginning of these spectral collections increase in approximately 20 minute intervals from Figure 8(a) to (b) to (c). The fine lines are due to gas phase water and these decrease in intensity with time after the commencement of the desorption. The spectra also include features due to gas phase nitric acid which vary in intensity but which are present at all three times. The identity of the features as due to nitric acid was confirmed by removing the lines due to water; the resulting spectrum is presented as Figure, with peaks at 1710, 1304, and 879 cm 1 confirming the identity of the nitric acid, HNO 3 at a concentration of ~55ppm. The large negative peaks are due to NO and NO 2 in the subtraction spectrum used. TABLE 1. Summary of FTIR experiments and findings FTIR Experiments Inputs Output Comment Mode 1 absorption Present in the reactor (on page 8) exit gas: NO 1000 ppm SO ppm O 2 5% H 2 O added as 150mL liquid (batch) 240 ppm N 2 O (from FTIR) Specific to Air Products Sour Gas Compression process (with presence of SO 2 ) Pressure 5 bar Residual SO 2, NO, NO 2 Mode 2 degassing of liquid after absorption (on page 9 10) NO 1000 ppm SO 2 nil O 2 5% H 2 O added as 150mL liquid (batch) Pressure 25 bar De gassing: Air 3 LPM H 2 O vapour from reactor Present in liquid off gas during degassing: Simulating conditions of the high pressure condensate removal stage at the COP CPU (after cooler), where liquids exit the system Mode 3 desorption of condensed acid (on page 12 13) NO 1000 ppm SO 2 nil O 2 5% H 2 O sat vapour after reactor Pressure 5 bar NO 2 Trace HONO No HNO 3 Present in the desorbing gas stream: Simulating conditions after the lower pressure condensate removal at the COP CPU (intercooler), where water vapour and NOx gases are present Desorption: 5% O 2 in N 2 HNO 3 11

12 a 2.0 Absorbance b c Wavenumber (cm -1 ) Figure 8: FTIR spectra of desorption gas with time from commencement of desorption increasing in approximately 20 minute intervals from (a) to (b) to (c) 12

13 cm cm 1 879cm 1 Absorbance Wavenumber (cm -1 ) 1000 Figure 9: FTIR Difference spectra after subtraction of water lines from acid desorption experiment (from Figure 4b above). Peaks at 1710, 1304, and 879 cm 1 confirm the identity of HNO 3 at a concentration of ~55ppm. 800 Conclusions Additional experiments made on the UoN small pressure rig were undertaken with NO x and mixed NO+SO x injection using a combined gas analysis system (FTIR/NO x /N 2 O) with Table 1 summarising the experiments. It was found that the variable amounts of water vapour had an over riding influence on baseline FTIR measurements which required experimental deviations from previous methods. Overall, this work has identified N 2 O being formed during the injection of a mixed acid gas stream in similar quantities to those measured by the dedicated N 2 O analyser. It has also confirmed the presence of HONO at trace levels when released during de gassing of the liquid phase from the 25 bar condition. This indicates that the measured NO and NO 2 traces are likely real gas concentrations evolved from desorbing out of the depressurised liquid. This also suggests that the 1 hour residence time at 25 bar is sufficient to oxidise dissolved HONO formed in the NOx absorption process. The FTIR also confirmed the presence of HNO 3 desorbing out the back sections of the reactor at concentrations similar to those measured with the NO x analyser. This indicates that for this specific experimental condition the NO 2 trace measured with the NO x analyser is reflective of real HNO 3 concentrations. The presence of high HNO 3 concentration in the gas phase represents a significant safety hazard during maintenance and unplanned shut downs of the high pressure circuit. It also confirms the previous theory on potential avenues for NOx capture in an oxyfuel compression system. 13

14 References 1. Spero, C., Callide Oxyfuel Project Lessons Learned, Report to the Global CCS Institute, oxyfuel project lessons learned, accessed, 14/09/ Stanger, R. and T. Wall, ANLECR&D project Milestone Report 4.1 Products formed from gas impurities in oxyfuel derived CO2 compression: Emissions on depressurisation, stability, disposal and utilisation potential March Nelson, P.F. and P.S. Bray, Formation of nitrogen containing gases under high pressure during compression of OxyFuel gas. ANLECR&D PROJECT : Gas quality impacts, assessment and control in oxy fuel technology for CCS: Part 2. Mercury removal with SO3 in the fabric filter and with NOx as liquids in CO2 compression. 2015, Report of a subcontract from the University of Newcastke to Macquarie University. p Li, H., et al., Gas quality control in oxy pf technology for carbon capture and storage a literature review, Report to Xstrata Coal Low Emissions Research and Development Corporation Wall, T. and R. Stanger, Gas quality impacts, assessment and control in oxy fuel and Gas quality impacts, assessment and control in oxy fuel: part 1, webinar to Global CCS Insitute, 21 July Linak, W.P., et al., Nitrous oxide emissions from fossil fuel combustion. Journal of Geophysical Research: Atmospheres, (D6): p

SO x /NO x co-removal during compression of oxy-fuel flue gas

ANLECR&D project 6-215-234 SO x /NO x co-removal during compression of oxy-fuel flue gas Milestone 4.1 Report of ANLECR&D Project 6-215-243 on: Products formed from gas impurities in oxyfuel-derived CO